Occurrence of Plesiomonas shigelloides in surface water: relationship with faecal pollution and trophic state.

Plesiomonas shigelloides is an autochthonous inhabitant of freshwater and has been associated with several waterborne outbreaks of acute gastro-enteritis. One of these outbreaks occurred in the summer of 1990, at a recreational beach near Amsterdam. In the summer of 1991, a survey was conducted to examine the presence of P. shigelloides in recreational fresh waters and marine water in the Netherlands. During the survey became clear that the specificity of current methods for the isolation and enumeration of P. shigelloides in water is low and requires extensive confirmation to obtain accurate data. P. shigelloides was ubiquitously present (30 of 42 samples positive) in fresh waters, but absent in sea water. Geometric mean Plesiomonas densities ranged from 140-340 per 100 ml; the range for individual positive samples was 33-840 per 100 ml. The Plesiomonas density was significantly correlated with index parameters for the trophic state (Secchi depth and chlorophyll A) and for faecal pollution (Escherichia coli). In winter, only 1 of 8 sites harboured P. shigelloides and in low numbers (4.4 per 100 ml). The ubiquitous presence of P. shigelloides in freshwater in the recreational season may result in cases of gastro-enteritis among bathers.